The present invention concerns magneto-optical recording media capable of overwriting by a single-beam, a recording method and an overwriting method, as well as an magneto-optical recording device using the media and, more in particular, it relates to magneto-optical recording media comprising a plurality of rare earth transition metal alloy magnetic layers, a recording method and an overwriting method as well as a magneto-optical recording device using the media.
Magneto-optical recording media having an exchange coupled magneto-optical bilayer in the prior art have been described in, for example, Japanese Patent Laid Open Sho No. 62-175948 and have a cross sectional structure as shown by a schematic view of FIG. 1. That is, on a transparent substrate 1 such as made of glass and provided with pre-grooves for tracking, are formed a dielectric layer 2 such as made of silicon nitride to a thickness of about 90 nm, a first magnetic layer 3 such as made of TbFeCo to a thickness of about 100 nm, a second magnetic layer 4 such as made of TbDyFeCo to a thickness of about 150 nm and a protection layer 5 such as made of silicon nitride to a thickness of about 200 nm being laminated in this order.
The dielectric layer 2 has a function of putting an incident laser beam on the side of the substrate 1 to multiple reflection at the inside of the layer and increasing the rotation of a polarization plane (Kerr rotation) caused in the first magnetic layer 3. The protection layer 5 has a function of protecting the first magnetic layer 3 and the second magnetic layer 4 against corrosion due to oxidation or the like. The second magnetic layer 4 is magnetically exchange-coupled with the first magnetic layer 3 and used for conducting single beam overwriting as described below.
Description will then be made to the principle of overwriting referring to FIG. 2. The Curie temperature (Tc.sub.1) of the first magnetic layer 3 is made lower than the Curie temperature (Tc.sub.2) of the second magnetic layer 4, and the coercivity of the second magnetic layer 4 is made smaller than that of the first magnetic layer 3. Accordingly, only the direction of magnetization 7b in the second magnetic layer 4 can be unified in one direction irrespective of magnetization direction 7a in the first magnetic layer 3 by merely applying an initializing magnetic field 8 which is greater than the coercivity in the second magnetic layer 4 and smaller than the coercivity in the first magnetic layer 3 by a permanent magnet or the like (FIG. 2(a), (b)).
When a laser beam of a relatively low intensity is irradiated to such a magneto-optical recording medium, since the temperature of the first magnetic layer 3 exceeds its Curie temperature, magnetization in the first magnetic layer 3 is eliminated (FIG. 2(c)), and the direction of the magnetization 7a in the first magnetic layer 3 is aligned with the direction of the magnetization 7b in the second magnetic layer 4 in the subsequent cooling (FIG. 2(e)). Further, when a laser beam of a relatively high intensity is irradiated, since the temperature of the second magnetic layer 4 exceeds its Curie temperature, magnetization in each of the first magnetic layer 3 and the second magnetic layer 4 is eliminated (FIG. 2(d)). Then, the direction of the magnetization 7b in the second magnetic layer 4 is aligned with the direction of a recording magnetic field 9 applied from the outside by a permanent magnet or like other means in the subsequent cooling, and the direction of the magnetization 7a in the first magnetic layer 3 is aligned with the direction of the magnetization 7b in the second magnetic layer 4 as the cooling proceeds further (FIG. 2(g)). Accordingly, by modulating the intensity of the laser beam, the direction of the magnetization 7a in the first magnetic layer 3 can be reversed optionally, making the single-beam overwriting possible.
Not-overwriting recording or erasing is also possible by using this magneto-optical recording medium. In this case, when recording is applied to an area in which information is not recorded (FIG. 2(a)), a laser beam of a relatively large intensity is irradiated without applying an initializing magnetic field to turn the state shown in FIG. 2(d) into that shown in FIG. 2(g). In the case of erasing, an erasing magnetic field 9' in the direction opposite to the recording magnetic field is applied as shown in FIG. 2(h) and then a laser beam of a relatively large intensity is irradiated to attain the state shown in FIG. 2(a).